Constructed Wetland Treatment of Tile Drainage - New Zealand
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New Zealand Constructed Wetland Treatment of Tile Drainage New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 1
New Zealand Constructed Wetland Treatment of Tile Drainage Authors: Chris C. Tanner James P. S. Sukias Charlotte R. Yates Acknowledgments: This guide was developed with funding from the Foundation for Research, Science and Technology (FRST) under Contract No. CO1X0304. The guidelines are based on information from a range of field and experimental studies funded through this and preceding FRST contracts; by the New Zealand dairy industry (Dairy Insight, Dexcel, Fonterra, Dairy Research Institute); and by the Department of Conservation. We thank NIWA and AgResearch staff who have assisted with monitoring, Paul Champion (NIWA) for advice on weed control, and the farmers who have allowed trials to be carried out on their farms, and assisted with wetland construction and management. Photos are mainly those of the authors. Additional photos provided by Rohan Wells, Paul Champion, Mary deWinton (NIWA). Citation: Tanner, C.C.; Sukias, J.P.S.; Yates, C.R. (2010). New Zealand guidelines: Constructed Wetland Treatment of Tile Drainage. NIWA Information Series No. 75 National Institute of Water & Atmospheric Research Ltd. New Zealand Guidelines for NIWA Information Series No. 75 Constructed Wetland © NIWA 2010 Treatment of Tile Drainage ISSN 1174-264X www.niwa.co.nz http://www.niwa.co.nz/our-science/freshwater/tools/tile-drain-wetland-guidelines Contact for more information: National Institute of Water & Atmospheric Research Ltd (NIWA) Chris C. Tanner (c.tanner@niwa.co.nz) PO Box 11115, Hillcrest, Hamilton 3251, New Zealand © 2010 – National Institute of Water & Atmospheric Research Limited. All rights reserved. Unless otherwise stated, the contents of this publication (including without limitation all text, diagrams, photographs, compilation of data or other materials) are subject to copyright. Copyright is owned by the National Institute of Water & Atmospheric Research Limited (“NIWA”). This publication may not be reproduced, adapted, or distributed, in whole or in part, or used for any commercial purpose without NIWA’s express written permission. While NIWA has used all reasonable endeavours to ensure that the information contained in this publication is accurate, no expressed or implied warranty is given by NIWA as to the completeness or accuracy of the information provided or that the guidelines contained herein will be appropriate for all situations. As the publication and information contained herein is of a general nature NIWA recommends that you seek specific advice, either from NIWA or other relevant industry professionals, regarding your intended use(s). Accordingly, you agree that if you use, or rely upon, any information within this publication NIWA will not be liable for any claim, loss, cost or damage incurred or suffered whatsoever. NIWA also recommends that prior to construction of a wetland you contact your local consenting authority to determine compliance with the Resource Management Act and/or all applicable local or regional regulations and plans. Graphic design: Aarti Wadhwa New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 2
Contents Purpose ........................................................................................................................................................................................................................... 1 Background ................................................................................................................................................................................................................... 2 Getting started ............................................................................................................................................................................................................. 5 Stage 1 Choose location …………………………………………………………………………………………………… 6 Scenario 1: End of tile drains, alongside surface drains and stream ............................................................................ 7 Scenario 2: End of surface drains supplied by tile drains ................................................................................................. 8 Stage 2 Design ……………………………………………………………………………………………………………… 9 A. Size Constructed wetland size and treatment performance ................................................................................................... 10 Calculating the wetland area required ................................................................................................................................... 11 B. Layout ................................................................................................................................................................................................. 12 C. Embankments and sealing .......................................................................................................................................................... 13 D. Shape Constructed wetland shape ........................................................................................................................................................ 14 Alternative constructed wetland shapes ............................................................................................................................... 15 Constructed wetland layout example ..................................................................................................................................... 16 E. Inlet structures ................................................................................................................................................................................. 17 Wetland width up to 8 m ............................................................................................................................................................. 18 Wetland width 8–30 m .................................................................................................................................................................. 19 Wetland width over 30 m ........................................................................................................................................................... 20 F. Sediment traps Where wetland may receive sediment-rich inflows ........................................................................................................... 21 New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 3
Contents G. Outlet and water-level control structures ............................................................................................................................. 22 H. Phosphorus retaining additives .............................................................................................................................................. 24 Stage 3 Construction ……………………………………………………………………………………………………… 25 A. Timeline/planning ......................................................................................................................................................................... 26 B. Construction day and beyond ................................................................................................................................................... 27 Stage 4 Establish plants …………………………………………………………………………………………………… 28 A. Role of plants ................................................................................................................................................................................... 29 B. Planting zones ................................................................................................................................................................................. 30 C. Plant selection Key species for the flooded zone .............................................................................................................................................. 31 Typha orientalis (raupo, bulrush) ....................................................................................................................................... 32 Baumea articulata (mokuautoto, jointed twig-rush) .................................................................................................. 33 Eleocharis sphacelata (kuta, tall spike-rush) ................................................................................................................... 34 Schoenoplectus tabernaemontani (kapungawha, soft-stem bulrush) .................................................................. 35 Key species for shallow-water margins and embankments ........................................................................................... 36 D. Planting and establishment ....................................................................................................................................................... 38 Stage 5 Maintenance ……………………………………………………………………………………………………… 41 A. Requirements during wetland establishment .................................................................................................................... 42 B. Requirements after wetland establishment ......................................................................................................................... 43 Appendices ………………………………………………………………………………………………………………… 44 Appendix 1: Specialist wetland plant suppliers in New Zealand ..................................................................................... 44 Appendix 2: Resources for wetland plant identification and management ................................................................. 45 Appendix 3: Herbicides for weed control during wetland establishment and maintenance ................................. 46 References ………………………………………………………………………………………………………………….. 47 Glossary …………………………………………………………………………………………………………………….. 48 New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 4
Purpose This guide provides information on how to design and construct wetlands to intercept and treat tile drainage flows from rain-fed grazed pasture in New Zealand. The focus is on removal of nitrate-nitrogen loads. Information to estimate potential nitrogen removal is based on the results of field studies and modelling. What is the purpose of The New Zealand Guidelines for Constructed Wetland Treatment of Tile these guidelines? Drainage are intended to guide farmers, farm advisors, rural contractors, and regional council staff to appropriately locate, size, design, and construct effective treatment wetlands. They also provide guidance on wetland planting, weed control, and maintenance in order to achieve effective treatment of tile drainage. Why build a constructed Tile drainage can act as a significant route for nutrient losses, particularly wetland to treat tile of nitrogen, from intensively grazed pastures to waterways. Current annual drainage flow? nitrogen losses in tile drainage flows from intensively grazed dairy pastures in New Zealand commonly range from 20–60 kg N per hectare, predominantly as nitrate. This quantity of nitrogen can pose a significant threat to water quality in sensitive receiving streams, rivers, lakes, and estuaries. Alternative design and management of tile drainage systems is required in these situations to integrate environmental goals with traditional agricultural water and soil management goals. Constructed wetlands offer a potentially cost-effective, practical option for intercepting highly variable tile drainage flows to reduce nutrient losses from pastures, before they reach receiving water bodies. They should ideally be employed in combination with good fertiliser, grazing, and effluent management practices. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 1
Background Constructed wetlands have been recognised in many countries as an effective technology for treatment of tile drainage waters. As a best management practice, they capitalise on natural processes to reduce pollution from incoming water, while enhancing local biodiversity values. What information are The recommendations in these guidelines are based on the results of these guidelines based farm-scale trials and modelling carried out by NIWA based on five years of monitoring in the Waikato, and three and four years of monitoring in on? Northland and Southland, respectively (Sukias, J.P.S. et al. (2006), Tanner, C.C. et al. (2005a), Tanner, C.C. et al. (2005b); Tanner & Sukias, (2010)). In combination with international experience, these results have allowed the quantification of nitrogen removal in relation to wetland size and design, providing realistic guidance on performance expectations for land managers and regulators. The constructed wetland performance predictions presented apply primarily to tile-drained areas of key dairying regions in Northland, Waikato, Bay of Plenty, Taranaki, and Manawatu, with rainfall in the range of 800–1400 mm per year. Estimated differences in performance for Southland and Otago, where there are also significant areas of tile-drained dairy pastures, are also given. What are constructed Natural wetlands have been called the “kidneys of the landscape” because of their ability to store, assimilate, and transform contaminants lost from the land, wetlands? before they reach waterways. Constructed wetlands attempt to mimic these natural systems to treat through-flowing waters. Generally shallow surface- flow wetlands are the most appropriate type of constructed wetland system for treatment of farm tile drainage. They are similar to natural marshes and swamps, in which water flows through shallow flooded beds of emergent aquatic plants such as raupo (Typha orientalis). New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 2
Background In some cases, naturally occurring wetlands on farms can be used to intercept and treat farm drainage. Constructed wetlands have the advantage that they can be created where they are required, avoiding the risk of contamination and degradation of high quality natural wetland habitats. How do constructed Flows are dispersed and slowed down when they enter wetlands, promoting wetlands work? settling and deposition of suspended particles. Wetlands are also highly productive environments for plants and beneficial microbes which break down, assimilate and cycle nutrients, organic matter, and associated pollutants, transforming them into less harmful forms. In particular, denitrifying bacteria provide an important means for sustainably removing nitrate-nitrogen by converting it into nitrogen gas that is released back into the atmosphere. What environmental Rainfall patterns, soil water status, groundwater levels, soil properties, factors affect drainage system design, and land-management practices affect the flows and performance? contaminant loads generated in farm tile drainage. Consequently, tile drainage flow rates can vary from a trickle to a deluge, and contaminant concentrations fluctuate widely from hour to hour and day to day. Drainage flows can also vary significantly between sites, seasons and years, and across different regions of the country. Depending on the relative size and design of the wetland, the resulting residence time (average amount of time drainage water spends in the wetland) may also vary widely. The biological removal processes operating in wetlands are most efficient when residence times are long and water temperatures are high. Performance tends to be poorer during cold, wet, winter periods. Such factors significantly affect the capacity of wetlands to remove nitrate-nitrogen and other contaminants. Because of this inherent variability, we have focused on providing estimates of long-term annual average percentage removal performance, and indicated the normal range of performance that can be expected in typical dairying regions of New Zealand. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 3
Background Situations these guidelines 1] Tile-drains conveying irrigated dairy effluent do not apply to: Tile drains can act as a rapid conduit for irrigated farm dairy effluent. Depending on soil characteristics and preceding soil water status, drainage from such areas may contain high concentrations of organic matter, nitrogen in the form of ammonium, and faecal microbes. Although constructed wetland treatment will likely improve such discharges (Tanner, C.C.; Kloosterman, V.C. (1997)), the sizing, design, and performance estimates provided in these guidelines do not apply to such situations. 2] Large-scale constructed wetlands The information presented in these guidelines is relevant for constructed wetlands up to 1 hectare (10,000 m2) in size in low gradient sites and stable soil conditions. For projects larger than this, and any situation where there is potential for land instability or flooding, it is recommended that you seek professional engineering advice. Compliance with RMA Please check with your local regional council regarding relevant rules and regulations. Construction of dams, culverts, or earthworks in or near waterways, in particular, may be subject to regional rules and/or require resource consent under the Resource Management Act 1991. Structural integrity Use of these guidelines does not in any way negate the need to seek professional engineering advice to ensure sound and appropriate design and construction, particularly for large-scale applications and where there is potential for land instability or flooding. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 4
Getting started It is critical to plan the implementation of the project, including timing of key stages, BEFORE starting your constructed wetland. There are five main stages to constructing a wetland: Stage 1 Choose location Stage 2 Design Stage 3 Construction Stage 4 Establish plants Stage 5 Maintenance New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 5
Stage 1 Choose location This section provides guidance for siting the constructed wetland. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 6
Stage 1Choose location Choosing a site to build your constructed wetland will depend on the configuration and existing functions of the available land and where the drainage system can best be intercepted. Likely areas include: What if the area available t at the downstream end of tile drains, alongside surface drains and streams, is limited? t at the end of surface drains that are supplied by tile drains. Tile drains rerouted to Providing access to the wetland to enable future maintenance It may be possible to is important. pass through CW before entering into stream. divide the wetland into separate sections with one flowing into another. Scenario 1: End of tile drains, For example, if a farm alongside surface drains and streams track divides the area, a An appropriate place to intercept tile drainage and construct subsurface pipe could link wetlands is alongside receiving drains or streams. This provides the sections on either side. valuable wetland habitat closely linked to waterways, and reduces potential interference with farming activities. A newly installed culvert linking two wetland sections. Intercepting a buried tile drain. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 7
Stage 1Choose location Surface drain Scenario 2: End of surface drains supplied by tile drains Constructed wetlands can also be used at the end of surface drains that collect and convey flows from several tile drains. Although predominantly receiving tile drainage, such surface drains are also likely to receive a component of surface drainage with higher particulate loads, particularly during intense rainstorms and extended wet periods. To deal with this additional sediment load from surface drainage, a sedimentation pond comprising an additional 10% of the wetland area should be included at the upstream end of the wetland (see page 21). To receiving water body Scraping off the turf before constructing a wetland at the end of a surface drain, supplied by tile drains. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 8
Stage 2 Design This section provides guidance for sizing and designing the constructed wetland. It has been broken down into nine sections: A. Size B. Layout C. Embankments and sealing D. Shape E. Inlet structures F. Sediment traps G. Outlet and water-level control structures H. Outlet protective structures I. Phosphorus retaining additives New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 9
Stage 2Design A. Size: Constructed wetland size and treatment performance How big do I build it? The size of the constructed wetland depends on your treatment goals and available land area. Generally the bigger the wetland the better the treatment achieved. The following graph, developed from multi-year data Use this chart as a guide from several different dairying regions, provides an estimate of average long-term nitrogen removal in relation to constructed wetland size. for determining how large your wetland needs to be to achieve your target 70% percentage nitrogen removal. Conversely, you For tile drain 60% removal flows from can use it to estimate the removal grazed pasture 53 ±15% percentage removal you in the North 50% removal nitrate-N can expect given your Island of nitrate-N New Zealand. wetland size. The areas (Performance annual 40% 40 ±15% of wetland are given in Southland removal annual and Otago as a percentage of the 30% Percentage of expected to Percentageof catchment area being be ~ 5–8% 22 ±10% lower across removal treated, therefore, you need 20% range, due to to measure the catchment lower average area that will drain into your temperatures). 10% constructed wetland. 0% 1% 2% 3% 4% 5% Percentageof Percentage drained of drained catchment catchment in in wetland wetland New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 10
Stage 2Design A. Size: Calculating the wetland area required Catchment area (ha) x % of catchment x 100 = Required wetland area (m2) 1 x 2.5 x 100 = 250 m2 The chart below provides examples of wetland surface area for various catchment sizes: Constructed Catchment % of wetland area area (ha) catchment (m2) 1 1 100 1 2.5 250 1 5 500 5 1 500 5 2.5 1250 A recently excavated, two stage, 250 m2 constructed wetland, with a mix of topsoil and subsoil returned into the base as the plant growth media. 5 5 2500 10 1 1000 10 2.5 2500 10 5 5000 20 1 2000 Note: One hectare = 10 000 m2. The required ‘area’ of the 20 2.5 5000 constructed wetland applies to the base area of the actual wetland; the embankments are additional to this. 20 5 10000 The same wetland planted with raupo (Typha orientalis) after two seasons’ growth. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 11
Stage 2Design B. Layout The drawings below show a simple constructed wetland layout. An inlet structure distributes the inflow water which then flows through the planted area before finally exiting through the outlet structure and discharging to the receiving waterway. Longitudinal view Embankment Inlet (rock rip-rap) Maximum planting ~0.8 m Wetland planting water level Outlet Discharge to 0.5 m stream/ 0.3 m drain Tile drain outlet 0.3 m Growth media (30 cm) 50:50 topsoil/subsoil Compacted clay lining (15 cm) Standard operating water level at low flow Inlet Top view Stream/ Outlet drain Excavating a wetland between a New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 12 surface drain and receiving lake.
Stage 2Design C. Embankments and sealing Why do we need to The embankment walls should be graded to provide a stable slope that will not erode or slump. This will seal the base of the generally require a slope between 2:1 and 1:2 length-to-width ratio depending on soil structural properties. constructed wetland? Appropriate vegetation of the embankments will help stabilise the banks. The base and lower 0.5 m of the embankments need to be sealed to retain the treated water and maintain To retain sufficient water sufficient moisture for survival of wetland vegetation. This can generally be achieved by compacting existing subsoils where the clay content is more than 10%. Compaction usually requires compression with a machine to keep the wetland such as a digger, a sheep-foot roller or similar. vegetation alive and Where clay is not available, suitable clay will need to be imported from off-site or use of a geosynthetic clay or maintain its treatment plastic liner (e.g., 250–500 micron low density polyethylene) considered. Plastic liners should be covered by at functions. least 200 mm of soil to prevent root penetration and provide protection from sunlight UV damage. This will require initially excavating to a deeper depth, fitting the liner, and then covering it with soil to form the base of the wetland. Cross-sectional view Width Standard low flow water level Maximum water level 1m 0.2 m 0.3 m 0.3 m Compacted clay lining Growth media (30 cm) 50:50 topsoil/subsoil mix Plastic liner used to make wetland water-tight in porous soils. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 13
Stage 2Design Rectangular shaped wetland D. Shape: Constructed wetland shape Tile drains rerouted to pass through CW before The shape of the wetland needs to provide the required entering into stream. area (see page 10) and length-to-width ratio to achieve effective treatment. An elongated rectangular shape is generally the simplest way to achieve this, but there are Why is shape a range of other options available to achieve a similar important? outcome. Recommended An elongated wetland Constructed wetland area Length:Width ratio shape with the inflow and outflow Up to 1000 m2 (0.1 ha) 3:1–5:1 promote even flow through the wetland. Very long narrow wetlands should be avoided because flow This reduces the velocities will become excessive during high-flow events. Length opportunity for short- Example of different length-to-width ratios for the same circuiting of flow or wetland area: creation of flow dead- zones, thus ensuring Constructed Length:Width Length Width maximum treatment wetland area ratio efficiency. 100 10 10:1 1000 m2 85 12 7:1 Width 65 15 4:1 Fence New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 14
Stage 2Design Free-form elongated shape D. Shape: Alternative constructed wetland shapes Tile drain An alternative way to increase the length-to-width ratio of a wetland is to use internal bunds to create a hairpin flow path. Highly serpentine designs with multiple internal bunds are not recommended as tight corners tend to cause flow “dead-zones” which reduce the treatment efficiency of the constructed wetland. The shape of constructed wetlands does not need to be rectangular. More natural flowing shapes can fit better into the landscape. However, to maintain good flow along the wetland and avoid dead-zones, we recommend maintaining length- to-width ratios between 3:1 and 10:1, depending on the total area of the constructed wetland (see page 14). Also, the width should not be varied by more than 20% along the length of the wetland. Hairpin shape nd B und Bu Free-form shaped constructed wetland receiving multiple inflows, being Tile drain built between a paddock and a lake. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 15
Stage 2Design D. Shape: Constructed wetland layout example This constructed wetland is approximately 900 m2 and receives water from a 4.5 hectare catchment. It is approximately 2% of the 1 catchment area, and has been divided into two sections to utilise the natural lie of the land and enable vehicle passage. Lower wetland Upper pond wetland 2 Spring: 8 months after construction and planting. 3 Autumn: 28 months after construction and planting. Excavating a wetland between a New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 16 surface drain and receiving lake.
Stage 2Design E. Inlet structures Why are inlet structures The performance of constructed wetlands is best when important? water flows evenly through the wetland utilising the full area available. Greater effort is required to disperse the inflow when Inlet structures help wetland width and size are larger. distribute the in-coming To contain the tile drain flow and provide support for the inlet water evenly across the structure, the final 2 m of the perforated tile drain entering the constructed wetland. inlet to the wetland should be rigid PVC pipe. This prevents erosion of The table below summarises the recommended inlet structures soil and plants near the to provide reasonable dispersal based on the constructed inlet. It also helps prevent wetland width. preferential flow channels Schoenoplectus growing in a constructed wetland. forming. This increases the Wetland width Inlet structure residence time of water in the wetland, enhancing Single central pipe with downward Up to 8 m facing bend discharging into pile of removal of nitrogen and coarse rock rip-rap. other pollutants. Inlet trench with discharge via 8–30 m submerged T-pipe. Note: In situations where the tile drains flow into a surface flow drain before reaching the wetland, it is recommended Split flow into two parallel wetlands of that a transverse trench or pond be incorporated at the inlet end of the wetland. This will act as a settling pond for More than 30 m lesser width using flow-splitting weir, suspended particles transported via the surface drains (see use combination of above structures. page 21). Mature constructed wetland. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 17
Stage 2Design E. Inlet structures: Wetland width up to 8 m Single pipe draining into pile of rip-rap Tile drain Downward facing elbow on the end of the drain pipe to distribute water via the rip-rap pile 50–80 mm rock rip-rap Water level A pile of 50–80 mm rip-rap under the subsurface drain is a simple method of slowing and distributing the inflowing water across Example of a single drain pipe and simple T-pipe, see page 19. constructed wetlands less than 8 m wide. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 18
Stage 2Design Submerged T-pipe E. Inlet structures: Wetland width 8–30 m Incoming tile drain pipe A ‘T’ connector and two 0.5 m pipe extensions should be fitted onto the end of the subsurface drain pipe to deflect flow laterally across an inflow trench or pond. Transverse deep-zone with submerged T-pipe inlet The transverse trench deep-zone length should be a minimum of 5 m and span the width of the constructed wetland. It should 8–30 be at least 1.2 m deeper than m the bottom of the main Top v constructed wetland ie w 0.5 m 0.5 m so that the water depth over 0.5 m the trench is 5m 0.5 m 1.2 m deep 1.5 m deep. The deep zone also serves as a sedimentation Wat pond trapping er le vel suspended particulates, 30 c Warratahs or timber can be used and may require periodic m to support the T-pipe. T-pipe must cleaning out. be fixed to supports to ensure even flow from each side. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 19
Stage 2Design Once wetland widths approach 30 m we recommend they are split into two parallel E. Inlet structures: Wetland width over 30 m wetlands by building a central bund to separate the two sides. The bund should Flow-splitting weir be a minimum of 0.8 m high. The inflowing Incoming tile Split flow into two parallel wetland drainage water will need to be split between drains the two wetlands. A well-designed and Flow-splitting weir maintained flow-splitting weir will be required T-pipe inlet structure to provide even flow to each side. To constructed wetlands Transverse trench or pond Dividing Example of flow-splitting weir set in a sump with lid removed; bund tile drainage water enters large chamber and is split between the two outlets via V-notches. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 20
Stage 2Design F. Sediment traps: Why settle out sediments? Where wetland may receive sediment-rich inflows Fine soil particles suspended High sediment inputs can silt up shallow wetlands reducing their effectiveness and longevity. If there is potential in drainage water increase for the constructed wetland to receive sediment-rich water from surface drains, flood flows, or other sources, an the turbidity of the receiving inlet pond is necessary. It should be 1.5 m deep and cover an additional 10% of the surface area of the wetland. waterways. This leads to The pond plays the same role as a transverse inlet trench, distributing the inflowing water evenly and allowing suspended sediments to be settled out and accumulated for later removal. smothering of the bottom, affecting aquatic organisms and recreational use. Longitudinal section of transverse inlet trench/pond Sediments can also be an important source of nutrients such as phosphorus. Minimum 5 m 0.3 m Deep zone 1.2 m Settled particles Sediment trap being excavated. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 21
Stage 2Design Outlet pipe protective structures G. Outlet and water-level control structures Timber chamber Stand-pipe: The height of the outlet structure determines the depth of water retained in the wetland. If the width of the outlet zone is less than 8 m, a single adjustable standpipe, or adjustable weir (see page 23) is generally sufficient. If the width of the wetland at the outlet end is wider than 8 m, a final 5 m-long outlet trench is recommended, similar to those used for the inlet zone. This improves collection and transmission of Vertical 20 mm flows from across the width of the wetland, increasing its hydraulic efficiency. An alternative option is to have timber gaps multiple outlet pipes or weirs at 6–8 m intervals across the width of the wetland outlet. However, these need slats to be carefully adjusted and maintained to ensure even outflow. Outlet pipes should be of similar or greater diameter to that of the combined inflow pipes supplying them. This ensures adequate drainage at high inflows, reducing the risk of overflowing the wetland (see note of caution, page 23). An upturned PVC standpipe Adjustable stand-pipe with outlet set 150 mm from the bottom is necessary for Normal operating depth the first six months of plant 150 mm Slotted plastic 200-litre drum establishment. Once the plants Plant establishment depth have established, an additional 150 mm 150 mm piece of pipe should be fitted onto the existing one Riser pipe to be installed to raise the water to an ideal after plants have established Attach drum to increase water depth. securely to operating level of 300 mm. waratahs Outlet protective structures: Outlet pipes or weirs should be protected from blockage by plant and other Outlet pipe debris. The two methods we recommend for protective structures for a stand-pipe are a slatted timber chamber or a slotted plastic drum. Cut slots or drill multiple holes of 40–60 mm in drum. The structure must be secured to fence posts or waratahs so it does not float away. To further prevent pipe blockage, a 1 m-wide filter zone of rip-rap can be incorporated around the structure. Removing debris from around protective structures is Note: Use lid over structure to block sunlight and thus minimise algal or weed growth on the outlet pipe. easier than unblocking a pipe. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 22
Stage 2Design G. Outlet and water-level control structures Why are outlet structures important? Weir: An alternative to using outflow pipes is to construct adjustable height weirs using concrete and/or treated timber as shown below. Ensure that low water levels can be maintained during plant establishment, Outlet structures provide and can be adjusted later. Care is needed to ensure stop logs are sealed so that water does not leak out around the edges, leaving the wetland plants dry between flow events. A triangular V-notch can be added in the centre control of the water level of the weir to enable flow estimates. This requires addition of a staff gauge to measure the height of water within the wetland. Water behind the weir. For further information see: www.engineeringtoolbox.com/weirs-flow-rate-d_592.html depth is especially important during plant establishment; Adjustable weir Removable if it is too deep, the plants timber or will float out or be drowned metal before they establish. stop-log set in slot. 150 mm 150 mm Wooden outlet weir structure. Note of caution: Overflow is a threat if unusually heavy rainfall occurs that generates inflows that exceed the capacity of the outflow pipe, or the outlet is blocked. The risk of overflow is greater in situations where the constructed wetland receives a significant amount of surface flow as well as tile drainage flow. In these cases you should consider installing a high-flow bypass- an additional higher level outlet pipe or an area of rip-rap on Outlet weir with one stop-log in place to control water level. the constructed wetland embankment where the water can safely spill out. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 23
Stage 2Design Other P-sorbing materials: A range of P-sorbing materials potentially suitable for H. Phosphorus retaining additives improving P retention in constructed wetlands have been identified; these include: allophanic Reduction of phosphorus (P) losses from farms is also important in many New Zealand waterways. Wetlands clays, lime, alum, smelter slag, and some volcanic are effective at settling out and retaining particulate phosphorus loads. However, in subsurface tile-drain tephras (Ballantine, D.J.; Tanner, C.C. (2010)). flows, dissolved forms of P generally predominate. The capacity of wetlands to remove these dissolved forms These materials are currently being assessed in of P is limited, with the main removal mechanisms being adsorption to soil minerals, uptake by plants, and laboratory trials with a view to field-testing. accumulation within organic matter. Adsorption of P under waterlogged conditions is limited unless additional P-sorptive media are added to the wetland soils and/or used as porous filter materials. There is also potential for P release from constructed wetlands if topsoils with high P status are used as growth media. It is difficult to predict the P release characteristics of normal pasture soils once subjected to water-logged conditions in constructed wetlands. For farms with P-retentive allophanic and pumice soils, P loss via drainage systems is unlikely to be a significant issue and topsoils are likely to be relatively P-retentive under wetland conditions. Constructed wetland planted with sedges and rushes. We recommend the following measures to increase P retention in constructed wetlands treating tile drainage, particularly in P-sensitive *Note: The use of subsoils with lower organic matter content catchments: is likely to reduce N removal performance during the first few years of operation. This will be overcome as wetland plant litter 1. Where possible use a 50:50 mix of subsoil from the B or C horizon accumulates in the wetland providing a sustainable ongoing mixed with topsoil as the growth media in the base of the wetland. supply of organic matter for denitrifying bacteria. Further This mix will reduce the overall P status of the wetland soil and studies are currently underway and recommendations will introduce more P-retentive material to the soil. If allophanic soils or be reviewed when further information becomes available. subsoils can be brought onto the site from nearby areas, these should **See following website for general distribution of be used in preference and/or combined with the local soil*. allophanic soils in New Zealand: http://soils.landcareresearch.co.nz/contents/ 2. Additionally, incorporate agricultural lime at a rate of 0.5 kg per m2 and follow links: into the top 50 mm of soil**. Soil Names à Soil classification (NZSC) à Soil Order Cyperus ustulatus New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 24
Stage 3 Construction This section provides guidance for building the constructed wetland. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 25
Stage 3Construction A. Timeline/planning Things to consider during the pre-construction stage: 1. Do you need permission? Check with your regional council that this is a permitted activity and for any specific restrictions or rules that need to be followed. 2. Identify: An experienced earthmoving contractor and make sure they know what is required to construct a wetland as outlined in this guide. 3. Make sure conditions are The soil should be dry enough for the digger to properly work the soil sufficiently dry for earthmoving: materials and avoid getting stuck. Excavating a constructed wetland. 4. Pre-order plants: It is wise to identify a supplier of wetland plants and speak to them about the types, quantities and timing of what you need several months in advance. This way they should be available when you need them (see Appendix 1). 5. Purchase: Purchase supplies for building the inlet and outlet structures. 6. Planting: The ideal time to plant is spring/early summer when there is still enough water coming out of the drains to supply the constructed wetland. It also gives the plants a head start to establish themselves before winter, improving their chances of survival. 7. Supplementary water supply: Often the best time of year for construction is in the summer when it is too dry for planting. If this is the case, you must have a supplementary water source available to keep the wetland moist to allow the plants to establish (see Stage 4). Otherwise planting should be delayed until conditions are suitable. Mix of topsoil and subsoil placed into bottom of excavated constructed wetland. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 26
Stage 3Construction B. Construction day and beyond 1. Before the digger arrives, peg out exactly where you want the constructed wetland to be dug. 2. The turf layer should initially be scraped off and disposed of. The topsoil should then be removed and set aside in a separate pile for later return as a growth media. 3. Excavate the wetland to the required depth. Rectangular constructed wetland planted with raupo. If clay is available, compact the base of the wetland, otherwise seal with the appropriate material (see page 13). 4. Once excavated, lined and shaped, cover the base with the set-aside 50:50 mix of topsoil and subsoil to a depth of 30 cm. Ensure that this growth media is only lightly packed down, suitable for planting into. 5. Install the inlet and outlet structures. 6. Plant the plants and flood with the appropriate amount of water (see Stage 4). Digger creating a pile of topsoil for later use as growth media. Starting excavation of a constructed wetland. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 27
Stage 4 Establish plants This section provides guidance for planting the constructed wetland including why, what, and how to plant. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 28
Stage 4Establish plants A. Role of plants When should I organise the plants? Constructed wetlands work best with a well-maintained, dense cover of emergent wetland plants. Plants are crucial to the functioning of constructed wetlands because they: You should start organising t provide the physical structure that supports the growth of the plants early on in your microbial biofilms, which are important in their functioning planning. It is likely you t produce litter as a source of organic matter for will need to order at least denitrification and other microbial processes a portion of your plants t shade the water surface to reduce algal growth from a specialist wetland t dissipate wind and wave action to reduce erosion nursery which may have t enhance wildlife and aesthetic values. to grow them for you. Carex species plants shading constructed wetland surface. You may also be able to harvest wetland plants for transplanting from existing wetland areas on your farm. Ensure you do not damage valuable natural wetlands, or transfer weeds along with the intended species, particularly in the soil around the plant roots. Raupo litter providing organic matter to support Schoenoplectus species dissipate wave action. denitrification. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 29
Stage 4Establish plants Key species* for: B. Planting zones Flooded zone t Raupo (Typha orientalis) There are three main regions within the constructed wetland: t Kapungawha (Scheonoplectus tabermontani) 1. the flooded zone t Mokuautoto (Baumea articulata) t Kuta (Eleocharis sphacelata) 2. the shallow margins Shallow margins and embankments 3. the embankments. t Harakeke, NZ Flax (Phormium tenax) Water level is the differentiating factor with the flooded zone being the wettest and the embankment being t Toetoe (native Cortaderia species) the driest. The plants in the flooded zone are primarily responsible for water treatment while the plants on the t Purei (Carex secta) margins and embankment stabilise the edges, help exclude weeds, and promote biodiversity. *Maori names for some native plants differ with region. We have attempted to use the most widely recognized name where possible. Constructed wetland cross-section illustrating planting zones Embankment Shallow-water margins Flooded zone New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 30
Stage 4Establish plants C. Plant selection: Key species for the flooded zone The tall, emergent plant species listed below grow well within the flooded zone of constructed wetlands. Native species adapted to your locality are recommended. Raupo is the plant of choice for treating farm drainage high in nitrate-nitrogen. It is easy to establish and fast-growing, producing abundant plant litter to stimulate nitrate-removing bacteria. It is relatively resistant to weed invasion and tends to dominate other species under nutrient-rich conditions. Alternatively, plant a combination of the other species as listed below. Plant Common Natural Depth Description species name range range raupo, bulrush 1.5–3 m tall. Dull green, erect leaves arising in clumps equivalent to: from green spongy rhizomes. Thick, cylindrical brown seed Throughout Typha orientalis cumbungi (Aus), heads (cat-tails) borne on tall shoots. Leaves die back 0–0.4 m New Zealand reed mace (UK), strongly in winter. Generally dominant emergent wetland cattail (N. America) plant in fertile lowland New Zealand swamps. 1.8 m tall. Green year-round. Dark green, ‘leafless’, stiff, mokuautoto, From Northland Baumea articulata cylindrical shoots with obvious joints when mature. Red- 0–0.4 m jointed twig-rush to Levin brown pendulous seed heads borne on separate fertile shoots. Throughout NZ (common in 0.8–1.3 m tall. Stout, bright green ‘leafless’, hollow Eleocharis kuta, tall spike-rush, North Island, shoots with transverse internal septa, arising from thick 0.2–0.6 m sphacelata spike-sedge uncommon in rhizome. Seed heads forming at tip of shoots. Canterbury) 0.6–1.8 m tall. Shoots die back over winter, except in kapungawha, Northland to northern coastal areas. Erect green to blue-green, ‘leafless’, Schoenoplectus soft-stem bulrush, Westland and cylindrical shoots with white central pith, arising from 0–0.4 m tabernaemontani lake clubrush Canterbury horizontal rhizome. Brown seed heads form tuft just below the shoot tip. Shoots die back in winter at inland sites. Baumea articulata (mokuautoto, jointed twig-rush). New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 31
Stage 4Establish plants Key flooded zone species: Typha orientalis (raupo, bulrush) New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 32
Stage 4Establish plants Key flooded zone species: Baumea articulata (mokuautoto, jointed twig-rush) New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 33
Stage 4Establish plants Key flooded zone species: Eleocharis sphacelata (kuta, tall spike-rush) New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 34
Stage 4Establish plants Key flooded zone species: Schoenoplectus tabernaemontani (kapungawha, soft-stem bulrush) New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 35
Stage 4Establish plants C. Plant selection: Key species for shallow-water margins and embankments These species are suitable for planting in shallow water and on embankment slopes to reduce bank erosion and weed ingression, and enhance plant and habitat diversity. For identification of these species, see ‘Wetland Plants in New Zealand’ (Johnson and Brooke, 1989). Plant Common Natural Planting Description Comments Photo species name range position purua grass, 1–1.8 m tall. Bolboschoenus kukuraho, Northland to Leafy sedges with stems, Common in coastal areas. Fast-growing Shallow water fluviatillis and ririwaka, river Westland and (triangular in cross- in spring and early summer, dies back to 0.3 m depth. B. medianus bulrush, Canterbury section), emerging from over winter. Provides seasonal diversity. marsh clubrush woody, bulbous tubers. 1–1.5 m tall. Moist lower Establish initially in moist conditions Drooping harsh embankments or shallow water, can grow in deeper Throughout Carex secta purei, makura tussocks forming trunk- and shallow water if gradually acclimatised. Classic New Zealand like base when mature. water to New Zealand plant of wetland and Green year-round. 0.2 m depth. stream margins. Other Carex spp.; Moist lower Taller-growing species mentioned are especially 0.5–1.5 m tall. embankments Throughout likely to be the most robust and able C. germinata, rautahi, carex Harsh leafy sedges. and shallow New Zealand to compete with weeds. Valuable for C. lessoniana and Green year-round. water to wildlife. C. virgata 0.2 m depth. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 36
Stage 4Establish plants C. Plant selection: Key species for shallow-water margins and embankments Plant Common Natural Planting Description Comments Photo species name range position toetoe (New Zealand Different 1.5–3 m tall. Upper & lower Useful, hardy plant suitable for bank Cortadera richardii, native species species Coarse green tussocks, embankments stabilisation and screening. Ensure C. fulvida, only, not to be common with tall feathery to water edge invasive introduced pampas species C. toetoe confused with in different flower heads borne on and surrounds. are avoided. introduced regions cylindrical stems. pampas grasses) Upper Tall-growing embankments ti kouka, Throughout soft-stemmed tree Classic New Zealand tree common in Cordyline australis and cabbage tree New Zealand bearing tufts of fibrous wet soils. surrounding leaves. areas. Northland to 0.5–1 m tall. Moist lower toetoe upoko- Canterbury Harsh pale-green leaves embankments Tolerates dry periods. Suitable for Cyperus ustulatus tangata, giant and Fjordland; in clumps, with emergent and shallow wetland margins and embankments, umbrella sedge mainly coastal seed-bearing leafy water to and shallow water. and lowland umbells. 0.2 m depth. 1–3 m tall. Upper & lower Does not generally establish well in Robust clumps of tough harekeke, Throughout embankments continuously flooded conditions. An Phormium tenax robust leaves. Tall dark New Zealand flax New Zealand to water edge important traditional plant for Maori brown to black flower and surrounds. and important habitat for wildlife. heads. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 37
Stage 4Establish plants D. Planting and establishment Care is needed to ensure rapid establishment of wetland plants and maximise the survival rate before weeds invade. Below are some tips for successful planting: 1. Timing of planting Most wetland plants do not grow much during winter and in many the above-ground portions die back over this period (e.g., raupo). Planting should, therefore, ideally take place in spring or early summer (September–December) to promote rapid establishment and enable growth of a tall dense cover that can outcompete weeds. However, planting at this time is often difficult in practice, because ground conditions remain too wet for construction early in the season. Planting Schoenoplectus into flooded zone. Later planting in summer (January–February) is possible if larger plant grades are used and a supplementary water source is available to keep the wetland moist. Planting later in the season, or without such supplementary care is not recommended. Instead, it is better to wait until the following spring to undertake planting. 2. Pre-planting weed and pest control If the area is weedy initially or has been left for any time, it will need manual weeding, or spraying with a translocated herbicide such as glyphosate. If large masses of herbicide-killed plant material are subsequently flooded they will decompose creating de-oxygenated conditions detrimental to plant establishment. It is therefore advisable to control weeds at low levels (to avoid build-up of biomass) and/ or physically remove any accumulated dead plants. Useful resources to help identify and control weeds are noted in Appendix 2 and Appendix 3. Pukeko and Canada Geese, in particular, can be very destructive to newly planted wetlands and may need controlling during early plant establishment. There are a range of bird deterrents that may be useful including gas bangers and flashing lasers. If you intend to dissuade birds by shooting, you will need a hunting permit from The Fish and Game Council and possibly also a special permit to extend control Tray of Schoenoplectus ready to be transplanted. beyond the open game-bird hunting season, www.fishandgame.org.nz/Site/Regulations/default.aspx. New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage 38
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